1. Field of the Invention
The present invention relates to a printer having an adjusting apparatus of head gap between a print head and a print sheet supported on a platen. In particular, the invention relates to the the printer in which the head gap can be correctly adjusted in case that the print head commences to move for printing after being maintained in nonuse state for a long time subsequent to that the printer is powered on.
2. Description of the Related Art
Generally, in a printer, it has come to be necessary to adjust a head gap between a print head and a print sheet supported on a platen, in order to maintain a high printing quality coresponding to various thickness of the print sheet. Thus, conventionally, various kinds of printers having a head adjusting device by which the head gap between the print head and the print sheet supported on the platen is properly adjusted corresponding to the thickness of the print sheet, are proposed. In U.S. Pat. No. 4,990,004, for instance, a printer having an automatic head gap adjusting device and an operator-controlled head gap adjusting device is disclosed. In such the printer, the head gap between the print head and the print sheet is automatically or manually adjusted according to the selected mode by a mode selector. Here, as shown in FIG. 1 of that U.S. patent, the print head 14 is mounted on a carriage 16, which has an integrally formed hollow cylindrical slide 17 slidably engaging a support shaft 18 parallel to the platen 10. And the slide 17 and the support shaft 18 are axially movable relative to each other.
Generally, lubricant is filled in a gap formed between the cylindrical slide 17 and the support shaft 18 so that the both the cylindrical slide 17 and the support shaft 18 can rotate relative to each other. Based on the above construction, the print head 14 is advanced to and retracted from the platen 10 according to forward and backward rotation of the support shaft 18 which is rotated by a driving mechanism connected to an eccentric end portion 26 formed on one end of the support shaft 18.
However, in general, the lubricant existing between the cylindrical slide 17 and the support shaft 18 has a characteristic to solidify itself if it is left for a long time as it is. Therefore, in case that the carriage 16 is moved for printing after being maintained in non-moving state for a long time, it is possible that the cylindrical slide 17 and the support shaft 18 are temporarily adhered and fixed each other due to solidification of the lubricant.
And in case of the print sheet thickness detection, the print head 16 is advanced and pressed to the print sheet with relatively small driving force so that a pressing mark by the print head 14 does not remain on the print sheet. Accordingly, when adhering or fixing force between the cylindrical slide 17 and the support shaft 18 is larger than the driving force for advancing the print sheet, the carriage 16 cannot be advanced to the print sheet.
Therefore, if, under the above condition, detecting of the print sheet thickness is conducted in order to calculate the optimum head gap, detecting error occurs because the cylindrical slide 17 and the support shaft 18 cannot rotate each other, therefore, the optimum head gap cannot be obtained. As a result, correct printing by the print head 14 cannot be conducted. In particular, if the print head 14 is a wire print head, print wires installed in the print head cannot reach to the print sheet supported on the platen 10, thus, correct printing cannot be conducted.
Here, there is no problem in a case that the printer is again utilized after powered off, since, in such case, the carriage 14 executes an initial operation in which the carriage 14 is retracted from the platen 10 to a standard position in an advance/retraction direction and further moved along the platen 10 to a standard position in a left/right direction, therefore, adhering or fixing between the cylindrical slide 17 and the support shaft 18 caused by solidification of the lubricant is removed.
However, in general usage of the printer, there is a case that printing of one page or so is conducted after powered on, and thereafter, the printer is left in nonuse state for a long time (several ten minutes). In this case, since the carriage 14 is stopped at a predetermined position for gap adjusting, the gap adjusting operation is immediately conducted in response to sheet feeding order from control device without moving of the carriage 14. In such case that the gap adjusting operation is conducted after the printer is left in nonuse state for a long time, the lubricant existing between the cylindrical slide 17 and the support shaft 18 is possibly solidified when the gap adjusting operation is conducted. According, detecting error in the gap adjusting will occur as mentioned above.
Further, for instance, in U.S. patent application Ser. No. 07/849,972, now U.S. Pat. No. 5,047,956 filed by the same applicant of the present invention, a printer having a gap adjusting apparatus for a print head is disclosed. In such printer, the gap adjusting apparatus similar to the above gap adjusting apparatus is utilized. Therefore, the same problem in the above printer yet exists if the gap adjusting is conducted after the printer is left in nonuse state for a long time subsequent to power on thereof.